questions about exp 12

[Robert Zellmer]

I've received a few questions about exp 12.  Here are the questions
and answers:

1)  For the density of H2O in part B what if my temperature is between
        those listed in the table on page 85?

Here are your two choices:

      a)  You can look up the density of water in the CRC manual. You can
          find a link to the on-line CRC on my web site (either from my 
homepage
          or the "Helpful Tidbits" link on our 122 webpage (not 
Carmen).  This
          is in Section 6: Fluid Properties, Standard Density of Water.  
There the
          density is listed at every 0.1 C to 7 decimal places.

      b) You can do a linear interpolation between the nearest two 
temperatures.
          In this case this may not give the best result because for the 
particular
          temp given of 15.8 C is between 10 and 20, a ten degree range.  It
          does work pretty well for the densities at temps between 20 
and 30 since
          those are listed at every one degree.  Even for temps in this 
range the
          error doing the linear interpolation using the info in the 
manual compared
          to that given in the CRC is only about 0.015 %.  This isn't 
going to effect your
          final calculated experimental MW in a significant way.

          By the way, what's a "linear interpolation"?  That means you 
take the
          two points (the two density and temp points) and you fit them 
to a line
          (i.e. get an eqn for a linear line).  You can use this eqn and 
the temp.
          you have to then find the density at your temperature which is 
between
          the two temperatures you used for the linear interpolation fit.

2) For Part D you are supposed to determine the molecular formula from the
      emp. form. by finding the ratio, MW/EFW (molec. wt. divided by 
emp. form.
      wt.).  The manual tells you to round down, even if you get 
something like 1.9.

      This can lead to two problem cases.

      a) The ratio is less than 1.  Check to make sure you did the 
calculations
          correctly for the MWs (Part C) and got the correct emp. form.  
If so
          you most likely made an experimental error (maybe waited too long
          when reweighing the flask after cooling it so some of your sample
          vaporized and escaped from the flask).  You can't round down 
to zero
          so you have to round up to 1 (molecular and emp. formulas are 
the same).

      b)  You round down as told and you get the wrong molecular formula.
          How would you know the formula might not be correct?  When you
          have a compound containing C and H or C, H and O the MOLECULAR
          formula MUST have an EVEN number of H atoms.  You can have an odd
          number of H atoms in an empirical formula.  However, You can 
NOT have
          an ODD number of H atoms in the MOLECULAR formula so a molec.
          formula such as C3H7 can not be correct (this is fine for an 
emp. form.).
          Perhaps you got this because your ratio in Part D was 1.8 and you
          rounded down to 1 (as told to do in the manual).  In this case 
you should
          really round up to a ratio of 2.  That would give C6H14.  I 
can't say that's
          correct.  You may still have done something else wrong. 
However, I can
          say the molecular formula can't be C3H7 so reporting it as 
C6H14 would
          be better.

3)  In Part C, what if two of the MWs are really close and one of the 
MWs is really
      different than the other two?  What can you do?

      a)  You can leave out the one which is really different. 
Technically, you should do
          an error analysis as explained in appendix F and at the 
following link on my web
          page ("Laboratory" link),

*Treatment of Numerical Data (Error Analysis, sig. fig., graphing)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/App_D_122_lab_manual.pdf>

      This may apply for other exps as well (exp 16 is another example 
with three trials
      for determining a MW).

4) What are reasonable values for "a" and "b" in the VDW eqn?

      Remember what these constants represent.  There is a table on page 
83 of the
      manual and Table 10.3 in Chapter 10 of the textbook which have "a" 
and "b"
      values for several substances.

5)  Getting a negative "a" value.  Does this make sense?

      Think about what "a" stands for in the VDW eqn.  It's about the 
attractive forces
      between particles which cause the measured pressure to be LOWER 
than the
      ideal pressure.  Look at the eqn and think about whether it could 
be negative.

      Remember, you are solving for "a" by simply rearranging the eqn.  
ALL the error
      in every value we substitute into the eqn winds up in "a". This is 
not the best way
      to find the "a" value.

Dr. Zellmer
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